Pushing the Limits of Organometallic Redox Chemistry with an Isolable Mn(−I) Dianion

We report an incredibly reducing and redox-active Mn^-I dianion, [Mn(CO)₃(Ph₂B(^tBuNHC)₂)]^2- (NHC = N-heterocyclic carbene), furnished via 2e^- reduction of the parent 16e^- Mn^I complex with Na⁰ or K⁰. Cyclic voltammograms show a Mn^0/-I redox couple at −3.13 V vs Fc^+/0 in tetrahydrofuran (THF), −3.06 V in 1,2-dimethoxyethane, and −2.85 V in acetonitrile. The diamagnetic Mn^-I dianion is stable in solution and solid-state at room temperature, tolerating a wide range of countercations ([M(2.2.2)crypt]⁺, [M(18-crown-6)]⁺, [ⁿBu₄N]⁺; M = Na, K). Countercation identity does not significantly alter ¹³C NMR spectral signatures with [ⁿBu₄N]⁺ and Na⁺, suggesting minimal ion pairing in solution. IR spectroscopy reveals a significant decrease in CO stretching frequencies from Mn^I to Mn^-I (ca. 240 cm^-1), consistent with a drastic increase in electron density at Mn. State-of-the-art DFT calculations are in excellent agreement with the observed IR spectral data. Moreover, the Mn^-I dianion behaves as a chemical reductant, smoothly releasing 1e^- or 2e^- to regenerate the oxidized Mn⁰ or Mn^I species in solution. The reducing potential of [Mn(CO)₃(Ph₂B(^tBuNHC)₂)]^2- surpasses the naphthalenide anion in THF (−3.09 V) and represents one of the strongest isolable chemical redox agents.